1. Field of the Invention
The present invention relates to improvements in a method of burning solid fuel such as coal, oil coke, oil shale, etc. by means of a fluidized bed.
2. Description of the Prior Art
The known method of burning solid fuel by means of a fluidized bed will be described with reference to FIG. 6. In this figure, primary air is charged through a primary air feed port opening at the bottom of a fluidized bed 1 containing fluidizable material such as sand, limestone, etc. to fluidize such fluidizable material, and solid fuel to be burned such as coal or the like, is charged through a fuel charging port 3. The temperature of the fluidized bed 1 is controlled by flowing water or steam through a heat transfer tube 4 provided within the fluidized bed 1. In addition, in a free board 5 is disposed a convection heat transfer section 6, through which water or steam is made to flow to collect thermal energy from a waste gas or gas of combustion. In addition, for the purpose of suppressing the amount of NO.sub.x generated during combustion and suppressing the exhaust of CO, secondary air is charged through secondary air feed ports 7. Normally, the fluidized bed 1 is operated under a condition where an air ratio with respect to a theoretical amount of air for complete combustion resulting from the primary air is about 1.0 in order to suppress the generation of CO. This reason is because, as fluidized bed combustion is effected at a relatively low temperature of 800.degree.-900.degree. C., the temperature of the free board 5 becomes as low as 500.degree.-700.degree. C., and hence if solid fuel is burnt at the fluidized bed with a low air ratio of 1.0 or lower, then the inconvenience would arise that the generated CO would not be perfectly burnt even by means of the secondary air, and CO would be exhausted. Therefore, under a practical operating condition, the limit of lowering the air ratio with respect to a theoretical amount of air resulted from the primary air at the fluidized bed 1 is about 1.0. Hence the fluidized bed will not become a reducing atmosphere, and as a result, the amount of generated NO.sub.x will become large (150-250 ppm (equivalent to 6% O.sub.2)).
It is to be noted that unburnt ash sputtered from the fluidized bed is captured by means of a cyclone 8 or the like and stored in a hopper 9. For the purpose of enhancing efficiency of combustion, part of the captured unburnt ash is recirculated to the fluidized bed 1 through an unburnt ash feeder 10 and circulation piping 11 at a recycling ratio of 0.1-0.5 kg/kg coal, but the remainder of the ash is exhausted externally through an ash extraction port 12.
The unburnt ash is separated in the cyclone 8, and combustion exhaust gas is exhausted externally from a cyclone outlet 13.
According to the above-described method of burning by means of a fluidized bed in the prior art, the following performance is generally attained:
1 the amount of generated NO.sub.x is 150-250 ppm (equivalent to 6% O.sub.2), and
2 the efficiency of combustion is 90-95%.
The above-mentioned limitations of performance attainable by the combustion method in the prior art, are not satisfactory.